Some processes for animating a character involve creating an animation rig for the character. The animation rig defines a jointed structure within the character, often a group of bones and joints. Some animation rigs have other structures that can be deformed based on the state of one or more bones or joints. For example, a rig may be enveloped so that the skin of the rig deforms based on the state of the joints. As this type of rig moves, the skin changes shape based on changes in the size and shape of the muscles of the rig.